To reproduce:
1. create two bullet lists like

• list 1 item 1
• list 1 item 2
• list 1 item 3

• list 2 item 1
• list 2 item 2

2. copy the entirety of list 2
3. paste it between items 2 and 3

you'd expect to get:

• list 1 item 1
• list 1 item 2
• list 2 item 1
• list 2 item 2
• list 1 item 3

instead you get:

• list 1 item 1
• list 1 item 2
  • list 2 item 1
  • list 2 item 2
• list 1 item 3